1. Field of the Invention
The present invention is directed to the field of electrical control systems and more specifically to the area of selecting from various voltage levels for application to a voltage sensitive load.
2. Description of the Prior Art
In conventional automotive electrical systems, the power used to energize selected loads is supplied by an alternator type DC generator supplemented by a battery. Such systems generally provide a single regulated level of DC voltage that is the rectified sum of three phases of alternating current Produced in the stator windings of the alternator. The full wave rectified output of the alternator is then applied to the battery in parallel with the selected loads. Some of the loads selected in an automotive vehicle are voltage dependent; such as, motors, pumps, heaters and lights. In such cases where a certain amount of control is desired to vary the performance of the voltage dependent load, it is common to place resistance between the supply source and the load to provide a voltage drop in series with the load.
FIGS. 1 and 2 illustrate typical prior art applications of varying the speed of a variable speed D.C. motor 40 utilizing series connected voltage dropping resistors. In FIG. 1, an alternator 10 supplies a DC output voltage to a battery B, through a rectifier bridge assembly made up of diodes 12, 14, 16, 18, 20 and 22. A switch S-1 is between the fully rectified output of the alternator and the motor 40. S-1 is switchable between four positions: an OFF position at contact 30, a LOW speed position at contact 32, a MEDIUM speed position at contact 34, and a HIGH speed position at contact 36. With respect to the LOW speed position of switch S-1, a resistor 38 is connected in series between contact 32 and the motor 40. At the MEDIUM speed position of switch S-1, a resistor 39 is connected in series between contact 34 and motor 40. In the HIGH speed position, the switch S-1 connects contact 36 directly to the motor 40.
When switching between the OFF position and the LOW, MEDIUM or HIGH positions, S-1 Provides the full DC voltage level to the selected contacts. In the LOW speed position at contact 32, resistor 38 is selectively sized to drop a portion of the voltage. The remainder of the voltage is dropped across the motor 40, and it reacts accordingly to rotate at a Predetermined and relatively low speed. Resistor 39 is of lesser value than resistor 38 so that the switching of S-1 to the MEDIUM position provides a lesser proportion of the voltage being dropped across resistor 39 than when in the LOW position; and, therefore, the motor 40 is caused to react and rotate at a somewhat higher speed. At the HIGH setting, the full DC voltage output from the alternator is applied through the contact 36 directly to the motor 40 where the entire voltage is dropped across that motor. The motor then rotates at its highest speed when the switch S-1 is in that position.
The problem with the conventional resistor switching configuration shown in FIG. 1 is that power is wasted in the LOW and MEDIUM switch positions because of the voltage drop across resistors 38 and 39. The wasted energy is manifested in excess heat which, in most cases, needs to be dissipated from the vehicle.
A second prior art approach of providing variable voltage to a voltage sensitive load is shown in FIG. 2. In that case, a switch S-2 is combined with a variable resistor 37 to select a value of resistance between the voltage source and the motor 40 voltage sensitive load. In that case, the switch S-2 has an OFF position 31. The remainder of the adjustments to S-2 incorporate some value of the adjustable resistor 37 in series with the motor 40. As the switch S-2 is adjusted towards the HIGH position, less resistance is in series so that a higher proportion of voltage is dropped across the motor 40. Similar to the prior art configuration shown in FIG. 1, energy is wasted in this prior art configuration because of the unused power dissipated across the series connected resistance.